It certainly is.
It looks like either they did a really poor job of power supply design (other phones don't "suddenly shut down" and they don't have this "feature"), or that they're just throttling for the obvious reason: they want you to buy a new phone.
As for their protest, quoted verbatim here from their letter:
First and foremost, we have never — and would never — do anything to intentionally shorten the life of any Apple product, or degrade the user experience to drive customer upgrades.
...this is utter bullshit. They constantly stop allowing their OS upgrades to run on hardware that is perfectly capable of running those upgrades. They've been caught at this multiple times. My 3 GHz, 12/24 core, 64 GB Mac Pro "can't" be upgraded to MacOS 10.13, so says Apple. But in fact, if you flash the bios to say that it's a machine made one year later, it'll upgrade perfectly. And why shouldn't it? It's little, if any, different than that machine. Even if it was slightly different (other than the date flashed into the hardware), this is a company with many, many billions of dollars in the bank that made a decision to obsolete this hardware for only one reason: So that it would go long in the tooth before its time and put buying pressure on the owner. There's no other possible reason.
They threw the PPC emulation out the window for just as little reason (no, probably less.) They let all those user's software suddenly go obsolete for a reason that boils down to "weren't going to pay for the emulation any longer", again, when they had tons of cash to maintain the tech and users had tons of PPC software. I still support PPC software running on (very) old machines, specifically because there is no reasonable in-OS upgrade path that lets that stuff keep running. The irony is that the massive power of the machines we have now would make those apps run very well indeed — and we know Apple did this as a choice, not a need.
I have more examples. From apps they took out of the store because they had integrated the tech into a new phone, thereby removing the possibility of users of an older phone having the tech unless they upgraded — to severe bugs they leave mouldering in old versions of the OS while not allowing upgrades to the new version of the OS, Apple is a known serial offender of the "let's pressure the customer."
Apple is lying here. Flat-out lying. And caught at it.